Regulated DC power supplies are typically needed for most analog and digital electronic systems. Two major categories of regulated DC power supplies are linear power supplies and switching power supplies. Generally, in linear power supplies, to provide electrical isolation between an input and an output and to provide the output in a desired voltage range, a transistor (operating in its active region) is connected in series with a transformer, e.g., a 60 Hz transformer.
In switching power supplies, transformation of DC voltage from one level to another is accomplished typically with DC/DC converter circuits, such as a step-down (buck) or step-up (boost) circuit. Solid-state devices, such as transistors, are operated as switches (either completely ON or completely OFF) within these switching converters. Since the power devices are not required to operate in their active region, this mode of operation results in lower power dissipation. Furthermore, increasing switching speeds, higher voltage and current ratings of these power devices are some of the factors that have increased the popularity of switching power supplies.
Two dominant topologies within an isolated medium-power category of switching power supplies are the half-bridge and the two-switch forward converter topologies. Both the half-bridge and the two-switch forward converters employ two switching devices that are used to impress a voltage waveform across the primary winding of an isolation transformer. The half-bridge converter topology offers excellent utilization of the transformer core and windings, since the flux swings symmetrically in both directions and current flows through the windings for most of each cycle. If, however, both switches are ON simultaneously, even for a fraction of a microsecond, large cross-conduction currents may result. These large cross-conduction currents, may in turn, severely stress and possibly destroy the controllable switches. To prevent these destructive cross-conduction currents, half-bridge converters typically limit the duty cycles of the controllable switches to insure sufficient "dead time" between each switch being ON. This forces peak currents and the size of the major power train components to increase. Alternatively, high speed detection of turn-off of one switch before allowing the opposite polarity switch to turn-on, is employed. This scheme, however, adds cost and complexity to the half-bridge regulation and gate drive circuitry. Even with these measures, the cross-conduction problem is difficult to completely mitigate due to circuit noise.
The forward converter, on the other hand, is inherently protected against cross-conduction, since the two controllable switches are designed to be turned ON simultaneously. The forward converter, however, is single-ended and, as a result, the transformer core flux excursions are only in one direction from zero. Consequently, only half of the transformer core's flux excursion capabilities are utilized. Furthermore, since current flows through the windings less than half of the time, the windings are also not efficiently utilized. Both of the above-described shortcomings contribute to requiring a substantially larger transformer and higher peak currents for a given power output than what would be typically employed in a half-bridge converter.
Accordingly, what is needed in the art is an improved half-bridge converter that mitigates the above-described problems. More specifically, what is needed in the art is a converter topology that has the efficient transformer utilization of the half-bridge converter with the robustness of the forward converter.